Explosive devices are increasingly being used in asymmetric warfare to cause destruction of property and loss of life, particularly in urban areas or against transportation facilities. These explosive devices can sometimes be disrupted but there often is not sufficient warning of an attack. This is becoming more so in a global scenario of suicide attacks and maximized mass casualties.
Explosive devices produce blast fragments emanating both from the device casing and from material close to the point of explosion, so called secondary fragmentation. In addition explosive devices produce shock waves, which can be characterized by having a rise time that is a virtual discontinuity in the physical properties of the gas through which it propagates. It is possible that acoustic waves may ramp up to form shock waves as higher pressure waves travel with a higher velocity than low pressure waves. However, for an explosive device the waves produced are always shock waves. Shock waves produce the highly damaging phenomenon known as blast. Shock waves travel at a speed related to their amplitude, higher pressures traveling faster than lower pressures, and the characteristics of a given medium. Once produced, the shock wave propagates outward from the source of the explosion obeying certain physical laws. These laws, the conservation of mass, momentum and energy, describe how the shock propagates through a medium and, importantly, how it propagates from medium to medium with the associated changes in velocity and pressure. Shocks propagating away from the source of the explosion will generally be expected to drop in pressure very rapidly. This is highly dependent on the area surrounding the explosion. Reflective barriers, tunnels, corners and many other structural features can reduce the rate at which the shockwave decays and, in some circumstances, locally increase pressures.
A shock propagating radially decays rapidly with distance as the energy is shared over an increasing surface area. Shocks travelling in a planar motion, such as in a tunnel, decay at significantly lower levels as they lose energy only at the edges where the wall and shock interface. This rate of pressure decay can be dramatically increased by placing material in the path of the shock. Materials that possess elements of differing shock impedance, the presence of phase boundaries and the ability to absorb energy by work done on producing irreversible changes within the material, are excellent shock pressure attenuators. Porous solid materials possess these qualities and are excellent attenuators of shock waves and therefore of blast. Gases and solid crystalline materials are inherently poor pressure wave attenuators.
Pressure waves can be reflected and diffracted by phase boundaries, such as liquid droplets or solid particulates suspended in air. These deflections serve to increase the distance that the wave travels by a process of multiple reflections and diffractions. Scattering and dispersion thus produce more attenuation because they smear the discontinuity leading the shock wave, the result of which is a significant drop in pressure in the material. This process has been shown to only provide a low level of attenuation over all, as the resultant acoustic wave emerging from the medium can ramp up again into a shock wave. Energy expended in accelerating the mass and in irreversible changes in the material, i.e., crushing, accounts for the majority of the attenuation. These mechanisms significantly reduce, or altogether eliminate, the pressure waves originally traveling in a specific direction.
Rebut, in French patent 2 573 511 discloses a partition or wall having high thermal and mechanical resistance comprising honeycombs into which are introduced compressible element(s) or which will impart properties of extensibility, inflammability, rigidity, or resistance to mechanical or thermal shocks. Examples of filler materials include aramide or compressible materials, or elastic materials. Other materials include foamed rubber, polyester, incombustible materials (for inflammability protection), which may include incombustible foamed rubber along with aramide or metallic materials. Mixtures of carbon/aramide can protect from about 600-700° C. Mixtures of carbon and ceramic protect up to about 2500° C., and ceramic alone protects up to about 3500° C. For rigidity, the cells may be filled with boron, carborundum, silica, etc.
Mazelsky, in U.S. Pat. No. 5,996,115, discloses flexible body armor made of a single layer of ceramic tiles adhesively attached to a flexible fragment-trapping jacket.
Gulbierz, in U.S. Pat. No. 3,801,416, discloses a flexible blast fragment blanket made of a plurality of layers of flexible blast-resistant material with blast-resistant plates embedded therein. Channels are located between the plates to impart flexibility to the blanket
Keenan et al., U.S. Pat. No. 6,289,816, discloses a water blanket for resting on pallets of ordnance to mitigate gas pressure loading from an inadvertent explosion of the ordnance. The blanket includes a pair of storage modules, and each module comprises a plurality of water storage compartments for water.
Gettle, in U.S. Pat. Nos. 5,225,622 and 5,394,786, discloses materials described as a flowable attenuating medium exhibiting aqueous foam characteristics which comprises solid particulate having bulk mechanical properties and flow properties of a fluid. These materials are produced as panels that are relatively rigid.
The most effective materials in attenuating acoustic waves and shocks are produced in flat panels. Most attenuating panels have, for ease of manufacture, been made as flat panels. When it is required to protect objects that are not flat, such as garbage receptacles and containers, flat panels do not provide adequate protection for non-flat surfaces, and the rigid material is not capable of being bent to conform to a curved surface. In many applications a blast attenuating material may be required for use outside. The material must be such that it is not affected by environmental conditions, such as water, snow, sleet, and other unfavorable conditions.